Non-volatile memory device and method for manufacturing the same

ABSTRACT

A non-volatile memory device and its manufacturing method are provided. The non-volatile memory device includes a tunneling oxide layer, a floating gate, a dielectric layer, and a control gate. The tunneling oxide layer is formed on a substrate. The floating gate is formed on the tunneling oxide layer, and includes a first polysilicon layer, a second polysilicon layer, and a nitrogen dopant. A grain of the first polysilicon layer has a first grain size, and a grain of the second polysilicon layer has a second grain size that is greater than the first grain size. The nitrogen dopant is formed in interstices between the grains of the first polysilicon layer. The dielectric layer includes a first nitride film, an oxide layer, a nitride layer, and an oxide layer conformally formed on the floating gate. The control gate is formed on the dielectric layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority of China Patent Application No.201710803384.4, filed on Sep. 8, 2017, the entirety of which isincorporated by reference herein.

BACKGROUND Field of the Disclosure

The present disclosure relates to a memory device, and in particular itrelates to a non-volatile memory device and a method for manufacturingthe non-volatile memory device.

Description of the Related Art

Flash memory has gradually become the mainstream of non-volatile memorybecause of its lower cost. Existing flash memory includes a floatinggate having a doped polysilicon layer. The grain size of the dopedpolysilicon layer is susceptible to subsequent high-temperatureprocesses, thereby increased. However, when the size of the grain thatis in contact with the tunneling oxide layer is larger, the dopant inthe polysilicon layer becomes more likely to gather at the interfacewith the tunneling oxide layer. As a result, the conductivity in someregions may abnormally increase, and problems such as over programmingand over erasing may occur.

So-called “over programming” means that electrons still move from thesubstrate through the tunneling oxide layer to the floating gate even ifno voltage is applied. So-called “over erasing” means that electronsstill move from the floating gate through the tunneling oxide layer tothe substrate even if no voltage is applied. When some regions of theinterface between the floating gate and the tunneling oxide layer havehigh conductivity, over programming and/or over erasing can easily occurin these regions. Both over programming and over erasing result inerrors in the operation of the non-volatile memory device. In addition,if over programming and/or over erasing occur, the variation of thethreshold voltage of the flash memory will become greater after adurability test. Therefore, good reliability and good durability cannotbe obtained.

With the recent trend of miniaturization of electronic products, thereis a demand for miniaturization of non-volatile memory devices.Moreover, the reliability and durability issues of existing non-volatilememory devices will become more severe in miniaturized designs.Therefore, there is still demand for non-volatile memory devices withhigh durability, high reliability, and high product yield.

BRIEF SUMMARY

The disclosure provides a non-volatile memory device. The non-volatilememory device includes a tunneling oxide layer on a substrate, and afloating gate on the tunneling oxide layer. The floating gate includes afirst polysilicon layer, a second polysilicon layer, and a nitrogendopant. The first polysilicon layer including a plurality of firstpolysilicon grains having a first grain size. The second polysiliconlayer including a dopant and a plurality of second polysilicon grainshaving a second grain size. The second grain size is greater than thefirst grain size. The nitrogen dopant is formed in the first polysiliconlayer and in interstices between the plurality of first polysilicongrains. The non-volatile memory device also includes a dielectric layeron the floating gate, and a control gate formed on the dielectric layer.The dielectric layer including a first nitride film conformally formedon and covering the floating gate, and a three-layer structureconsisting of a first oxide layer, a nitride layer, and a second oxidelayer structure sequentially and conformally formed on the first nitridefilm.

The disclosure also provides a method for manufacturing a non-volatilememory device. The method includes forming a tunneling oxide layer on asubstrate, and forming a floating gate on the tunneling oxide layer.Forming the floating gate includes performing a first deposition processto form a first polysilicon layer on the tunneling oxide layer,performing an ion implantation process to implant an impurity comprisingN₂ into a surface of the first polysilicon layer, and performing asecond deposition process to form a second polysilicon layer on thefirst polysilicon layer. The first polysilicon layer is an undopedpolysilicon layer, and the second polysilicon layer is a polysiliconlayer doped with a dopant. The method also includes performing a heattreatment process to form a plurality of first polysilicon grains havinga first grain size in the first polysilicon layer, and to form aplurality of second polysilicon grains having a second grain size in thesecond polysilicon layer. The second grain size is greater than thefirst grain size. The method also includes forming a dielectric layerformed on the floating gate, and forming a control gate on thedielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G show cross-sectional views of a non-volatile memory devicein accordance with some embodiments;

FIG. 2 shows a cross-sectional view of a non-volatile memory device inaccordance with other embodiments;

FIGS. 3A-3B show the experimental results of the concentration profilesof the dopants of the non-volatile memory devices in Test Example (A)and Test Example (B);

FIG. 4 shows the experimental results of the concentration profiles ofthe dopants of the non-volatile memory device in Test Example (C);

FIG. 5 shows the experimental results of the difference value of thethreshold voltages of the non-volatile memory devices in ComparativeExample (A) and Example (A).

DETAILED DESCRIPTION

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It should be notedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the relative dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion. In addition, the present disclosure may repeat referencenumerals and/or letters in the various examples. This repetition is forthe purpose of simplicity and clarity and does not in itself dictate arelationship between the various embodiments and/or configurationsdiscussed.

In the disclosure, the term “about” or “approximately” means in a rangeof 20% of a given value or range, preferably 10%, and more preferably5%. In the disclosure, if there is no specific explanation, a givenvalue or range means an approximate value which may imply the meaning of“about” or “approximately”.

In the disclosure, the unit “%” of the indicated content is “atomicpercentage (atom %)” or “ionic percentage (ion %)”. For example, if thecontent of the X component is 10% and the content of the Y component is90% in a material or structure, it means that there are 10× atoms (orions) and 90 Y atoms (or ions) per 100 atoms (or ions) in the materialor structure.

In some embodiments of the disclosure, a non-volatile memory device anda method for manufacturing the non-volatile memory device are provided.FIGS. 1A-1G show cross-sectional views of a non-volatile memory device100 in accordance with some embodiments.

Referring to FIG. 1A, a tunneling oxide layer 104 is formed on asubstrate 102. In some embodiments, the tunneling oxide layer 104 may beformed by thermal oxidation, but the invention is not limited to this.The substrate 102 may include an array region and a peripheral circuitregion (not shown) adjacent to the array region. For the purpose ofsimplicity, FIGS. 1A-1G show merely cross-sectional views of the arrayregion. The substrate 102 may be a semiconductor substrate. In someembodiments, the material of the substrate 102 may include silicon,gallium arsenide, gallium nitride, silicon germanium,silicon-on-insulator (SOI), other applicable materials, or a combinationthereof. In some embodiments, other structures may be formed in thesubstrate 102, for example, an N-type well, a P-type well, a P/Njunction, or an isolation structure.

In some embodiments, after the tunneling oxide layer 104 is formed, inorder to form a thin nitride layer on the surface of the tunneling oxidelayer 104, an optional nitrogen gas plasma treatment may be performed.The nitrogen gas plasma is electrically neutral as a whole, and itincludes various species of nitrogen, for example, cation (N⁺, N₂ ⁺),anion (N⁻, N₂ ⁻), free radical (N*, N₂*), neutral nitrogen atom (N), andN₂ molecules. These species of nitrogen have applicable energy togenerate a weak bonding force with the silicon atoms.

In some embodiments, after the tunneling oxide layer 104 is formed, anoptional high-temperature annealing process in a nitrogen-containing gasenvironment may be performed. The nitrogen-containing gas may includenitrogen oxides, such as, nitrogen monoxide (NO), nitrogen dioxide(NO₂), dinitrogen oxide (N₂O), dinitrogen trioxide (N₂O₃), dinitrogentetroxide (N₂O₄), or a combination thereof. The temperature of theannealing process may be 70-1200° C. In some embodiments, after thetunneling oxide layer 104 is formed, an optional nitrogen gas plasmatreatment may be performed, and then, an optional high-temperatureannealing process in the nitrogen-containing gas environment may beperformed. The threshold voltage of the non-volatile memory device 100may be improved by performing the nitrogen gas plasma treatment and/orthe annealing process. The details will be discussed in the followingparagraphs.

Then, a first deposition process is performed to form a firstpolysilicon layer 110 having a thickness T1 on the tunneling oxide layer104. The first deposition process may include a chemical vapordeposition (CVD) process, an atomic layer deposition (ALD) process,other applicable deposition process, or a combination thereof. In someembodiments, the first polysilicon layer 110 may be formed by performinga low pressure chemical vapor deposition (LPCVD) in a furnace.

The first polysilicon layer 110 is an undoped polysilicon layer, and itis used to be a buffer layer or a barrier layer. The first polysiliconlayer 110 may prevent the dopant (e.g., phosphorus or arsenic) inanother polysilicon layer deposited subsequently from diffusing into thetunneling oxide layer 104 and affecting the electron tunneling effect.Therefore, the threshold voltage of the non-volatile memory device 100may be improved. If the thickness of the first polysilicon layer 110 istoo small, the threshold voltage cannot be significantly improved. Onthe other hand, since the undoped polysilicon layer has a higherelectrical resistance value, if the thickness of the first polysiliconlayer 110 is too large, the electrical resistance value of thenon-volatile memory device 100 may be too high. Therefore, a higheroperating voltage may be required. As a result, the energy consumptionof the device may be increased and the durability of the device may bedecreased. Accordingly, the thickness of the first polysilicon layer 110may be controlled within an applicable range. In some embodiments, thethickness T1 of the first polysilicon layer 110 is 5-40 nm.

Still referring to FIG. 1A, an ion implantation process 115 is performedto implant an impurity comprising N₂ into the surface of the firstpolysilicon layer 110. As shown in FIG. 1B, after the ion implantationprocess 115 is performed, an N₂-doped thin layer 112 is formed on thesurface of the first polysilicon layer 110. The ion source used in theion implantation process 115 may have a high content of N₂ ⁺ ions. Insome embodiments, the ion source used in the ion implantation process115 is an N₂ ⁺ ion having a concentration that is equal to or greaterthan 99%. Therefore, it is advantageous to improve the threshold voltageof the non-volatile memory, and the reliability and durability of thedevice may be significantly improved. In other embodiments, the ionsource used in the ion implantation process 115 is an N₂ ⁺ ion having aconcentration that is equal to or more than 99.9% or substantially 100%.The ion source having a high content of N₂ ⁺ ions may be produced by anyapplicable methods. For example, nitrogen gas may be ionized in theionizer to produce ions having different mass/charge ratios (m/e). Then,the ions are separated from each other by an electric field or amagnetic field, and the N₂ ⁺ ions are focused to form an ion beam whichmay be used as an ion source for the ion implantation process 115. Inone embodiment, when the ion implantation process 115 is performed, theion implantation apparatus may provide secondary electrons which haveequal amounts of charge to neutralize the positively charged ion beam,and then, apply the ion beam onto the wafer. Therefore, the damage ofthe wafer caused by the accumulation of the positive ions in the wafermay be prevented. It should be realized that the above-described methodsare merely examples and are not intended to limit the invention.

Then, as shown in FIG. 1C, a second deposition process is performed toform a second polysilicon layer 120 on the first polysilicon layer 110and the N₂-doped thin layer 112. In some embodiments, the secondpolysilicon layer 120 may be a polysilicon layer doped with a dopant. Insome embodiments, the second polysilicon layer 120 may be a polysiliconlayer doped with an N-type dopant. In some embodiments, the N-typedopant may include phosphorus or arsenic.

The second deposition process may include a CVD process, an ALD process,other applicable deposition process, or a combination thereof. In someembodiments, the second deposition process may be a low pressurechemical vapor deposition performed in a furnace, and the seconddeposition process may include an in-situ doping process. As a result,the polysilicon layer may be deposited and doped with the dopantsimultaneously to form the doped second polysilicon layer 120. In someembodiments, in order to dope phosphorus into the second polysiliconlayer 120, phosphine (PH₃) is used as the dopant gas of the in-situdoping process.

In some embodiments, the thickness T2 of the second polysilicon layer120 is 20-200 nm. In preferable embodiments, the thickness T2 of thesecond polysilicon layer 120 is greater than the thickness T1 of thefirst polysilicon layer 110. In a preferable embodiment, the thicknessT2 of the second polysilicon layer 120 is at least twice as large as thethickness T1 of the first polysilicon layer 110.

Then, as shown in FIG. 1D, a first etching process is performed to forma plurality of first trenches 121 through the second polysilicon layer120, the N₂-doped thin layer 112, the first polysilicon layer 110, thetunneling oxide layer 104, and the substrate 102. The first etchingprocess may include a plasma etching process, reactive ion etching (RIE)process, other applicable etching processes, or a combination thereof.In some embodiments, in order to form the first trench 121 having ahigher aspect ratio, the first etching process is reactive ion etchingprocess

Referring to FIG. 1E, insulating material is formed in the firsttrenches 121, and a second etching process is performed to remove aportion of the insulating material. Accordingly, isolation structures122 are formed in the first trenches 121, and second trenches 123 areformed on the isolation structures 122. A plurality of floating gatesmay be isolated from one another by these isolation structures 122.

The isolation structure 122 having a single layer structure is shown inFIG. 1E. However, it should be realized that the embodiments are merelyfor the purpose of simplicity, and are not intended to limit theinvention. In other words, the isolation structure 122 may be a singlelayer structure or a multi-layer structure. Furthermore, the isolationstructure 122 may include silicon nitride, silicon oxide, siliconoxynitride, other applicable insulating materials, or a combinationthereof.

Further, a liner (not shown) may be formed on the inner wall of thefirst trench 121 before forming the insulating material in the firsttrench 121. In some embodiments, an oxide liner may be formed by ahigh-temperature thermal oxidation process. In such embodiments, thesecond polysilicon layer 120 experienced the high-temperature thermaloxidation process may include a plurality of second polysilicon grainseach having a second grain size greater than a first grain size of eachof first polysilicon grains in the first polysilicon layer 110.

It should be noted that after absorbing the heat energy in thehigh-temperature process, the phosphorous dopant in the secondpolysilicon layer 120 may be converted into a gaseous state and outgasoutward easily. As result, the concentration of phosphorus dopant may bedecreased, and the electrical resistance value of the second polysiliconlayer 120 may be increased. On the other hand, since the nitrogen dopantis formed in the first polysilicon layer 110 of the invention, theelectrical resistance value of the first polysilicon layer 110 isincreased. In addition, when the swollen phosphorus dopant outgasoutward after absorbing the heat energy, the hills-like protrusions maybe left on the surface of the second polysilicon layer 120, and anuneven surface may be left. As a result, the yield of the product may bedecreased. In particular, the impact on miniaturized device will becomemore serious. In order to reduce the whole electrical resistance valueof the floating gate, the doping dose of the phosphorus dopant may beincreased. However, if the doping dose of the phosphorus dopant isincreased, the problem of the above-mentioned protrusions may becomemore serious.

In order to solve the above problems, as shown in FIG. 1F, after theformation of the isolation structure 122, a first nitrogen gas plasmatreatment is performed at room temperature to conformally form a firstnitride film 131 on the surface of the second polysilicon layer 120 andthe surface of the isolation structure 122.

In the invention, by forming the first nitride film 131 at roomtemperature, the probability of outward outgassing of the phosphorusdopant may be remarkably reduced. Furthermore, even if the phosphorousdopant outgassed outward, the first nitride film 131 may block theoutgassing of the phosphorus dopant, and most of the phosphorus dopantmay be kept in the second polysilicon layer 120. Therefore, if the firstnitride film 131 is formed, the electrical resistance value of thesecond polysilicon layer 120 may be prevented from rising after thehigh-temperature process even if the doping dose of the phosphorusdopant is not increased.

In order to effectively block the outgassing of the phosphorus dopant,the thickness of the first nitride film 131 may be 1-5 Å (angstrom).

After the first nitride film 131 is formed, a three-layer structureconsisting of an oxide layer 132, a nitride layer 133 and an oxide layer134 is sequentially and conformally formed on the first nitride film131. The three-layer structure may be formed by using any applicablematerial or deposition process, such as, a high-temperature furnaceprocess. In some embodiments, the oxide layer 132 and the oxide layer134 may include silicon oxide.

Then, a second nitrogen gas plasma treatment is performed to conformallyform a second nitride film 135 on the surface of the oxide layer 134.The second nitrogen gas plasma treatment may be the same as or similarto the first nitrogen gas plasma treatment, and the second nitride film135 may be the same as or similar to the first nitride film 131.Therefore, the details will not be repeated here.

In some embodiments, in order to block the diffusion of the phosphorusdopant, the first nitride film 131 and the second nitride film 135 areformed below and above the three-layer structure, respectively.Therefore, the second polysilicon layer 120 may be maintained at astable electrical resistance value, and the variation of the thresholdvoltage may be reduced.

Furthermore, the first nitride film 131 and the second nitride film 135may reduce the equivalent oxide thickness (EOT). Therefore, a lowervoltage is required for programming/erasing. In other words, thethreshold voltage may be reduced, and the durability of the non-volatilememory device may be improved.

In addition, a portion closer to the edge of the tunneling oxide layer104 may be re-oxidized (also known as the bird's beak effect) in thesubsequent heat treatment, thereby the tunneling oxide layer 104 maybecome thicker. The first nitride film 131 and the second nitride film135 may prevent the bird's beak effect, and therefore, the operatingvoltage distribution of the non-volatile memory device may be moreuniform.

In order to improve the reliability and durability of the non-volatilememory device, the thickness of the second nitride film 135 may be 1-5Å.

Then, as shown in FIG. 1G the polysilicon material 140 is deposited onthe second nitride film 135 and filled into the second trench 123. Afterthe polysilicon material 140 is formed, other conventional processes(e.g., patterning the polysilicon material 140 to form a control gate)may be performed to accomplish the non-volatile memory devices 100. Thedetails of the conventional processes will not be described here.

Referring to FIG. 1G the non-volatile memory device 100 of the inventionmay include a substrate 102, a tunneling oxide layer 104, a floatinggate, a dielectric layer, and a control gate. The tunneling oxide layer104 is disposed on the substrate 102. The floating gate is disposed onthe tunneling oxide layer 104 and includes a first polysilicon layer110, a second polysilicon layer 120, and a nitrogen dopant. The secondpolysilicon layer 120 includes a dopant. The dielectric layer includes afive-layer structure that is conformally formed on and covers thefloating gate. The five-layer structure includes a first nitride film131, an oxide layer 132, a nitride layer 133, an oxide layer 134, and asecond nitride film 135. The control gate is disposed on the dielectriclayer.

The first polysilicon layer 110 includes a plurality of firstpolysilicon grains having a first grain size. The second polysiliconlayer 120 includes a plurality of second polysilicon grains having asecond grain size. The second grain size is greater than the first grainsize. The nitrogen dopant is formed and in interstices between theplurality of first polysilicon grains of the first polysilicon layer.

If the first grain size is too large, the interstices between the firstpolysilicon grains are so large that the dopant in the secondpolysilicon layer 120 is easily moved to be in interstices between thefirst polysilicon grains. As a result, the variation of the thresholdvoltage of the non-volatile memory device 100 is increased, and thereliability and durability of the device are decreased. In someembodiments, the first grain size is 1-70 nm. In some embodiments, thefirst grain size is 3-40 nm.

In the invention, a non-silicon dopant (e.g., nitrogen) is doped on thesurface of the first polysilicon layer 110. Without these dopants, thegrains of the first polysilicon layer may be bonded together during thesubsequent heat treatment process, thereby increasing the grain size ofthe grains. In contrast, due to these dopants between the grains, thebonding of the grains during the heat treatment process becomes moredifficult or even does not occur. That is, because of the ionimplantation process 115 of the invention, the grain size of the firstpolysilicon layer 110 may be prevented from being significantly affectedby the heat treatment process. Therefore, the first grain size issmaller than the second grain size.

If the nitrogen dopant is doped into the surface of the firstpolysilicon layer 110 by conventional nitrogen gas plasma, it ispossible to avoid the increase in the grain size of the firstpolysilicon layer 110 due to the heat treatment process. However, theamount of the N-type dopant (e.g., phosphorus) diffusing from the secondpolysilicon layer 120 to the first polysilicon layer 110 cannot besignificantly reduced. In this case, these phosphorus dopants willgather in the interstices between the grains of the first polysiliconlayer 110 or in the interstice between the first polysilicon layer 110and the tunneling oxide layer. Furthermore, the phosphorus dopantdiffused into the first polysilicon layer 110 may further diffuse intothe substrate 102. As a result, the conductivity of some areas will beincreased abnormally, and the problem of over programming and/or overerasing may occur.

After several methods have been tried, the inventors of the inventionhave found that the problem of over programming and/or over erasing maybe improved by using a high content of N₂ ⁺ ion as an ion source for ionimplantation process. Therefore, the threshold voltage of thenon-volatile memory device 100 may be improved, and the reliability anddurability of the device may be significantly improved.

The reasons of why N₂ ⁺ ions can improve the problem of over programmingand/or over erasing will be discussed in the following paragraphs. TheN₂ ⁺ ion is a monovalent cation formed by two nitrogen atoms. On theother hand, the N⁺ ion is a monovalent cation formed by one nitrogenatom. The mass of the N₂ ⁺ ion is relatively larger than the mass of theN⁺ ion. Therefore, in the first polysilicon layer 110, the diffusion ormovement of the neutralized dopant N₂ is more difficult, and theneutralized dopant N₂ may concentrate at the surface of the firstpolysilicon layer 110 and form a layer containing a high concentrationof N₂ (for example, the N₂-doped thin layer 112). Because of theN₂-doped thin layer 112, it is possible to prevent the grain size of thegrains of the first polysilicon layer 110 from being affected after theheat treatment process, and the diffusion of the phosphorus dopant maybe blocked more effectively. In other words, the N₂-doped thin layer 112may significantly reduce the amount of phosphorus dopant entering thefirst polysilicon layer 110. Therefore, the above-mentioned problem ofover programming and/or over erasing may be significantly reduced orsolved.

In contrast, if nitrogen is doped onto the surface of the firstpolysilicon layer 110 by nitrogen gas treatment, the species of nitrogenhaving lower mass (e.g., N⁺, N⁻, N* and N atom) will diffuse or move toa deeper position in the first polysilicon layer 110. Therefore, thenitrogen dopant cannot concentrate at the surface of the firstpolysilicon layer 110, and the concentration of nitrogen dopant at thesurface of the first polysilicon layer 110 may become lower. As aresult, the ability of the first polysilicon layer 110 to block thediffusion of the phosphorus dopant is poor, and the problem of overprogramming and/or over erasing may not be effectively improved.

In order to verify the above-mentioned deduction, the inventors of theinvention have conducted experiments and the results are shown in FIGS.3A-3B. FIGS. 3A-3B show the experimental results of the concentrationprofiles of the dopants of the non-volatile memory devices in TestExample (A) and Test Example (B).

The manufacturing process of Test Example (A) includes: an ionimplantation process is performed on a silicon substrate by using N⁺ions as an ion source; then, a phosphorus-doped polysilicon layer havinga thickness of 150 nm is deposited on the silicon substrate; then, ahigh-temperature annealing process is performed at 1050° C. and undernitrogen gas environment. The manufacturing process of Test Example (B)is the same as that of Test Example (A), except that the ionimplantation process of Test Example (B) is performed by using N₂ ⁺ ionsas the ion source. Test Example (A) and Test Example (B) were analyzedby secondary ion mass spectrometry (SIMS). The concentration profiles ofthe nitrogen dopant is shown in FIG. 3A, and the concentration profilesof the phosphorus dopant is shown in FIG. 3B.

Referring to FIG. 3A, the concentration of nitrogen dopant (N) in TestExample (A) had a peak at the depth of about 150 nm, which representsthat the dopant N was concentrated at the interface between the siliconsubstrate and the polysilicon layer. However, the nitrogen dopant ofTest Example (A) had a severe tailing phenomenon in the region having adepth of about 50 to 150 nm, which represents that the dopant N wasdriven by the stress of the growth of the grains in the polysiliconlayer after the heat treatment, so that a lot of the dopant N diffusedinto the polysilicon layer. On the other hand, the concentration ofnitrogen dopant (N₂) in Test Example (B) had a peak at a depth of about150 nm, and the variation of the dopant N₂ concentration before andafter this peak was extremely small, which represents that the dopant N₂was concentrated at the interface between the silicon substrate and thepolysilicon layer and hardly diffused to the polysilicon layer. In otherwords, the above-mentioned tailing phenomenon may be avoided by using N₂as the nitrogen dopant.

Referring to FIG. 3B, at a depth of about 200 nm, the concentration ofphosphorus dopant in Test Example (A) was about 10¹⁸ atoms/cm³, and theconcentration of phosphorus dopant in Test Example (B) was about 10¹⁷atoms/cm³. Furthermore, the concentration of phosphorus in Test Example(A) was significantly higher than that in Test Example (B) in the regionhaving the depth of about 170-220 nm, which represents that comparedwith nitrogen dopant (N), the diffusion of phosphorus dopant may be moreeffectively blocked by using N₂ as the nitrogen dopant.

From the above experimental results, it can be realized that when N⁺ions are used as the nitrogen dopant in the ion implantation process115, the nitrogen dopant may be attracted by the stress of the growth ofthe grains in the polysilicon layer thereon. Therefore, theabove-mentioned tailing phenomenon of the nitrogen dopant may easilyoccur in the second polysilicon layer. That is, the concentration ofnitrogen dopant at different depth ranges may be significantly uneven.Since the nitrogen dopant can inhibit the growth of polysilicon grains,if the concentration of nitrogen dopant is higher, the grain size ofpolysilicon will be smaller. Therefore, the above-mentioned tailingphenomenon of the nitrogen dopant may cause the grain size of the secondpolysilicon layer to become uneven. That is, the variability of thegrain size may be increased.

In contrast, performing the ion implantation process by using N₂ ⁺ ionsas the ion source may significantly reduce the amount of the phosphorusdopant entering the first polysilicon layer 110. Therefore, the problemof over programming and/or over erasing may be significantly improved orsolved. Accordingly, the threshold voltage of the non-volatile memorydevice 100 may be improved, and the reliability and durability of thedevice may be significantly improved.

Furthermore, in order to verify that the phosphorous dopant can befurther blocked by doping the nitrogen dopant in the tunneling oxidelayer 104 and the substrate 102, the inventors have conductedexperiments and the results are shown in FIG. 4. FIG. 4 shows theexperimental results of the concentration profiles of the dopants of thenon-volatile memory device in Test Example (C).

Test Example (C) includes the structure as shown in FIG. 1C and ismanufactured in accordance with the above-mentioned steps described inFIGS. 1A to 1C. The nitrogen gas plasma treatment and thehigh-temperature annealing process are performed after the formation ofthe tunneling oxide layer 104. FIG. 4 shows the concentration profilesof the nitrogen dopant and the phosphorus dopant in Test Example (C)analyzed by SIMS. In FIG. 4, the dotted line represents theconcentration profile of the nitrogen dopant, and the solid linerepresents the concentration profile of the phosphorus dopant.

As shown in FIG. 4, the concentration profile of the nitrogen dopantincludes the first peak P1, the second peak P2, the third peak P3, andthe fourth peak P4. The first peak P1 was located in the firstpolysilicon layer 110. The second peak P2 was located in the substrate102. The third peak P3 and the fourth peak P4 were located in the secondpolysilicon layer 120.

Referring to FIG. 4, the first peak P1 was located at the position ofthe surface of the first polysilicon layer 110, and the concentration ofnitrogen dopant at the first peak P1 was significantly higher than theconcentration of nitrogen dopant at the third peak P3, which representsthat most of the nitrogen dopant N₂ was kept near the interface of thefirst polysilicon layer 110 and the second polysilicon layer 120, andonly a very small amount of nitrogen dopant entered the secondpolysilicon layer 120. The third peaks P3 and the fourth peaks P4 werelocated at different depths in the second polysilicon layer 120. Theconcentration of nitrogen dopant at the third peak P3 is very close tothe concentration of nitrogen dopant at the fourth peak P4, whichrepresents that the above-mentioned tailing phenomenon of the nitrogendopant entering the second polysilicon layer 120 did not occur.

Referring to FIG. 4, because the above-mentioned nitrogen gas plasmatreatment and the above-mentioned annealing process were performed, theconcentration profile of the nitrogen dopant had a second peak P2 in thesubstrate 102 and another peak in the tunneling oxide layer 104 (notmarked). The concentration of nitrogen dopant at the second peak P2 isclose to the concentration of nitrogen dopant at the first peak P1.

Still referring to FIG. 4, the concentration profile of the phosphorusdopant dropped sharply in the tunneling oxide layer 104 and continueddecreasing in the substrate 102. It has been verified that if a highconcentration of nitrogen dopant is present in the tunneling oxide layer104 and the substrate 102, the phosphorus dopant may be further blockedto prevent the phosphorus dopant from entering the tunneling oxide layer104 and the substrate 102. Accordingly, the threshold voltage, thereliability and the durability of the non-volatile memory device 100 maybe further improved.

In some embodiments, in the non-volatile memory device 100 of theinvention, the concentration of nitrogen dopant at the third peak P3 isnot greater than the concentration of nitrogen dopant at the first peakP1 and the concentration of nitrogen dopant at the second peak P2.Therefore, the problem of over programming and/or over erasing may beimproved or solved. As a result, the threshold voltage of thenon-volatile memory device 100 may be improved, and the reliability anddurability of the device may be significantly improved.

In some embodiments, in the non-volatile memory device 100 of theinvention, the value of the concentration of nitrogen dopant at thesecond peak P2 divided by the concentration of nitrogen dopant at thethird peak P3 is in a range of 10²-10⁵. Therefore, the phosphorus dopantmay be prevented from entering the tunneling oxide layer 104 and thesubstrate 102. As a result, the first nitride film 131 of thenon-volatile memory device 100 of the invention may be used as a caplayer or a barrier layer to block the outgassing of the phosphorousdopant in the second polysilicon layer 120 after the high-temperatureprocess. In some embodiments, the nitrogen concentration of firstnitride film is 10²¹-10²³ atoms/cm³. In some embodiments, theconcentration of phosphorus dopant in the second polysilicon layer 120is 10²⁰-10²² atoms/cm³.

In some embodiments, in the non-volatile memory device 100 of theinvention, the value of the concentration of nitrogen dopant at thefourth peak P4 divided by the concentration of nitrogen dopant at thethird peak P3 is not greater than 1. Therefore, the uniformity of thegrain size of the second polysilicon layer is increased.

For two separated floating gates, because the volume (or the number) ofthe interstice between the first polysilicon grain may be different, theconcentrations of the nitrogen dopant and the electrical resistancevalues may be different. For example, if the grain size of the firstpolysilicon grain is greater than the width of the floating gate, theremay be no interstice between the grains in the first polysilicon layer110 of some floating gates. Such a floating gate may have a lowerconcentration of nitrogen dopant and a lower resistance. In other words,there may be uncontrollable differences in these floating gates. As aresult, the yield and reliability of the non-volatile memory device 100may be reduced. With the miniaturization of memory devices, this problemmay become more serious.

In order to improve the above-mentioned problems, in the non-volatilememory device 100 of the invention, the relative relationship betweenthe first grain size and the width of the floating gate may becontrolled as needed. As shown in FIG. 1G the width of the floating gateis represented by W1. In some embodiments, the ratio of the first grainsize to the width W1 of the floating gate is 0.05-0.95. In someembodiments, the ratio of the first grain size to the width W1 of thefloating gate is 0.35-0.75.

In addition, in order to further improve the performance of thenon-volatile memory device 100, the depth of the first nitride film 131of the non-volatile memory device 100 of the invention may be controlledwithin a specific range.

In some embodiments, as shown in FIG. 1G the maximum depth of the firstnitride film 131 is D1, the depth of the top surface of the firstpolysilicon layer 110 is the second depth D2, and the difference valueof the second depth D2 minus the maximum depth D1 is ΔD. In someembodiments, ΔD is a positive value, i.e., the lowest portion of thefirst nitride film 131 is higher than the top surface of the firstpolysilicon layer 110. If the value of the maximum depth D1 is toolarge, the first nitride film 131 is too close to the tunneling oxidelayer 104. As a result, it is highly probable that the problem of overprogramming will occur. On the other hand, if the value of the maximumdepth D1 is too small, the second trench 123 is too shallow and ΔD istoo large. As a result, it is highly probable that the problem of overerasing will occur. Accordingly, the variation of the threshold voltageof the non-volatile memory device 100 may be increased, and thereliability and durability of the device may be decreased. With theminiaturization of memory devices, these problems may become moreserious.

In order to address the above-mentioned problems, the relativerelationship between the depth of the first nitride film 131 and thedepth of the top surface of the first polysilicon layer 110 may becontrolled as needed. In some embodiments, the difference value ΔD is5-50 nm. In some embodiments, the difference value ΔD is 10-30 nm.

In order to further verify the advantages of the first nitride film, thesecond nitride film and the implantation of N₂ ⁺ ions, the inventors ofthe invention have conducted experiments. FIG. 5 shows the experimentalresults of the variations of the threshold voltages of the non-volatilememory devices in Comparative Example (A) and Example (A).

Example (A) is a non-volatile memory device manufactured in accordancewith the above-mentioned steps described in the FIGS. 1A to 1G and theresulting non-volatile memory device includes a structure as shown inFIG. 1G The non-volatile memory device of Comparative Example (A) wasmanufactured in accordance with a procedure similar to that of Example(A), except that Comparative Example (A) did not carry out an N₂ ⁺ ionimplantation and did not form a first nitride film and a second nitridefilm. The non-volatile memory devices of Example (A) and ComparativeExample (A) were subjected to programming/erasing operations for 10⁵cycles, and the threshold voltage was measured. The difference valuebetween the maximum value and the minimum value of the threshold voltage(hereinafter referred to as the difference value of the thresholdvoltages) is shown in FIG. 5.

In FIG. 5, if the difference value of the threshold voltages is larger,the variation of the threshold voltage is larger. In other words, thereliability of the non-volatile memory device is worse. On the otherhand, if the average difference value of the threshold voltages exceeds3000 mV, it represents that the device failed the durability test.

Referring to FIG. 5, the average difference value of the thresholdvoltage was about 3800 mV for the non-volatile memory device ofComparative Example (A). That is, the device failed the durability test.The average difference value of threshold voltage was about 2800 mV forthe non-volatile memory device of Example (A). That is, the devicepassed the durability test.

From the above experimental results, it has been verified that theproblems of over programming and over erasing may be significantlyimproved or solved in the non-volatile memory device 100 of theinvention. Therefore, the threshold voltage of the non-volatile memorydevice may be improved, and the reliability and durability of the devicemay be significantly improved.

FIG. 2 shows a cross-sectional view of a non-volatile memory device 200in accordance with other embodiments. FIG. 2 is similar to FIG. 1,except that the third polysilicon layer 150 is formed on the secondpolysilicon layer 120 before the first nitride thin film 131 is formed.The same elements as those in FIG. 1 are denoted by the same referencenumerals. For the sake of simplicity of explanation, the elements whichare the same as or similar to those in FIG. 1 are not repeated here.

In some embodiments, the supply of the dopant gas may be discontinuedafter the formation of the doped second polysilicon layer 120, and thein-situ deposition process (i.e., the third deposition process) may becontinued to form the undoped third polysilicon layer 150 on the secondpolysilicon layer 120.

Because the third polysilicon layer 150 has no dopant (e.g., aphosphorous dopant), the third polysilicon layer 150 may also serve as acap layer or a barrier layer to block the outgassing of the phosphorusdopant in the second polysilicon layer. Therefore, the protrusionsformed on the surface of the second polysilicon layer 120 due to theoutgassing of the phosphorus dopant may be decreased. As a result, thereliability and durability of the non-volatile memory device may befurther improved. In some embodiments, the thickness of the thirdpolysilicon layer 150 is 1-50 nm.

As described above, the advantages of the non-volatile memory device andthe method for manufacturing the non-volatile memory device inaccordance with the embodiments of the invention include at least:

(1) Using a high content of N₂ ⁺ ions as an ion source, the nitrogendopant is concentrated on the surface of the first polysilicon layer.Therefore, the problems of over programming and over erasing may besignificantly improved or solved, and the reliability and durability ofthe non-volatile memory device may be significantly improved.

(2) The formation of the first nitride film and the second nitride filmcan prevent the outgassing of the phosphorus dopant and decrease theprotrusions on the surface of the second polysilicon layer. Therefore,the variation of the threshold voltage of the non-volatile memory devicemay be decreased, and the reliability and durability of the non-volatilememory device may be improved.

(3) Nitrogen dopant may be optionally doped in the tunneling oxide layerand the substrate to further block the phosphorus dopant. Therefore, thethreshold voltage, reliability and durability of the non-volatile memorydevice may be further improved.

(4) The third polysilicon layer may be optionally formed to furtherprevent the outgassing of the phosphorous dopant and to further decreasethe protrusions on the surface of the second polysilicon layer.Therefore, the reliability and durability of the non-volatile memorydevice may be further improved.

(5) The ion implantation process using N₂ ⁺ ions as an ion source can beeasily integrated into existing non-volatile memory device processeswithout substantial modification or replacement of process and/orproduction equipment, thus the effect on the cost of production issmall.

Although the disclosure has been described by way of example and interms of the preferred embodiments, it should be understood that variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art) can be made herein without departing from the spiritand scope of the disclosure as defined by the appended claims.

What is claimed is:
 1. A non-volatile memory device, comprising: atunneling oxide layer formed on a substrate; a floating gate formed onthe tunneling oxide layer, wherein the floating gate comprises: a firstpolysilicon layer comprising a plurality of first polysilicon grainshaving a first grain size; a second polysilicon layer comprising aplurality of second polysilicon grains having a second grain size,wherein the second grain size is greater than the first grain size, andwherein the second polysilicon layer comprises a dopant; and a nitrogendopant formed in the first polysilicon layer and in interstices betweenthe plurality of first polysilicon grains; a dielectric layer formed onthe floating gate, wherein the dielectric layer comprises: a firstnitride film conformally formed on and covering the floating gate; and athree-layer structure consisting of a first oxide layer, a nitride layerand a second oxide layer sequentially and conformally formed on thefirst nitride film; and a control gate formed on the dielectric layer.2. The non-volatile memory device as claimed in claim 1, wherein thedielectric layer further comprises a second nitride film conformallyformed on the three-layer structure.
 3. The non-volatile memory deviceas claimed in claim 1, wherein a thickness of the second polysiliconlayer is greater than a thickness of the first polysilicon layer.
 4. Thenon-volatile memory device as claimed in claim 1, wherein the firstgrain size is 1-70 nm.
 5. The non-volatile memory device as claimed inclaim 1, wherein the floating gate has a width, and a ratio of the firstgrain size to the width is 0.05-0.95.
 6. The non-volatile memory deviceas claimed in claim 1, wherein the dopant is phosphorus, and the dopanthas a concentration of 10²⁰-10²² atoms/cm³.
 7. The non-volatile memorydevice as claimed in claim 2, wherein a thickness of the first nitridefilm is 1-5 Å, and a thickness of the second nitride film is 1-5 Å. 8.The non-volatile memory device as claimed in claim 1, wherein nitrogenconcentration in the first nitride film is 10²¹-10²³ atoms/cm³.
 9. Thenon-volatile memory device as claimed in claim 1, wherein the nitrogendopant is further formed in the second polysilicon layer, and aconcentration profile within the substrate, the first polysilicon layer,and the second polysilicon layer comprises a first peak, a second peak,and a third peak, wherein the first peak is located in the firstpolysilicon layer, the second peak is located in the substrate, and thethird peak is located in the second polysilicon layer, and wherein aconcentration of nitrogen dopant at the third peak is not greater than aconcentration of nitrogen dopant at the first peak and a concentrationof nitrogen dopant at the second peak.
 10. The non-volatile memorydevice as claimed in claim 9, wherein a value of the concentration ofnitrogen dopant at the first peak divided by the concentration ofnitrogen dopant at the third peak is in a range of 10²-10⁵.
 11. Thenon-volatile memory device as claimed in claim 9, wherein theconcentration profile within the second polysilicon layer furthercomprises a fourth peak, and a value of a concentration of nitrogendopant at the fourth peak divided by the concentration of nitrogendopant at the third peak is not greater than
 1. 12. The non-volatilememory device as claimed in claim 1, wherein the first nitride film hasa maximum depth D1, a top surface of the first polysilicon layer has asecond depth D2, and a difference value ΔD of the second depth D2 minusthe maximum depth D1 is 5-50 nm.
 13. A method for manufacturing anon-volatile memory device, comprising: forming a tunneling oxide layeron a substrate; forming a floating gate on the tunneling oxide layer,wherein forming the floating gate comprises: performing a firstdeposition process to form a first polysilicon layer on the tunnelingoxide layer, wherein the first polysilicon layer is an undopedpolysilicon layer; performing an ion implantation process to implant animpurity comprising N₂ into a surface of the first polysilicon layer;performing a second deposition process to form a second polysiliconlayer on the first polysilicon layer, wherein the second polysiliconlayer is a polysilicon layer doped with a dopant; and performing a heattreatment process to form a plurality of first polysilicon grains havinga first grain size in the first polysilicon layer, and to form aplurality of second polysilicon grains having a second grain size in thesecond polysilicon layer, wherein the second grain size is greater thanthe first grain size; forming a dielectric layer formed on the floatinggate; and forming a control gate on the dielectric layer.
 14. The methodfor manufacturing a non-volatile memory device as claimed in claim 13,wherein an ion source used in the ion implantation process is an N₂ ⁺ion having a concentration that is equal to or more than 99%.
 15. Themethod for manufacturing a non-volatile memory device as claimed inclaim 13, wherein forming the dielectric layer formed on the floatinggate comprises: forming a first nitride film on a surface of thefloating gate; and conformally forming a three-layer structureconsisting of a first oxide layer, a nitride layer, and a second oxidelayer on the first nitride film.
 16. The method for manufacturing anon-volatile memory device as claimed in claim 15, wherein forming thedielectric layer formed on the floating gate further comprises:conformally forming a second nitride film on the three-layer structure.17. The method for manufacturing a non-volatile memory device as claimedin claim 13, wherein the second deposition process comprises an in-situdoping process, and the in-situ doping process uses phosphorus as thedopant.
 18. The method for manufacturing a non-volatile memory device asclaimed in claim 13 that, after performing the second depositionprocess, further comprises: performing a third deposition process toform a third polysilicon layer on the second polysilicon layer, whereinthe third polysilicon layer is an undoped polysilicon layer.